If you already hold a nvq3 , I would suggest that you do the 2395. The 2394 is initial verification, the 2395 is periodic inspection and testing. You should have already covered initial verification in your NVQ3(If you dont have a nvq3 yet then this is your next step). So most sparks are going for the 2395. There are 3 exams for both of the above. Practical,on-line multi guess and closed book written. Alot of people fail the written exam , around only 30% are passing it(similar to the old 2391 pass rate). You will need to commit to revising the GN3 for BS7671 because if you dont your wasting your time and money.Also the centre I attended for my 2395 didnt do any practical at all. Which amused me as one of the exams is practical.They expect you to already know what your doing, and your just there to gain the qualification. It will mainly be down to you to get your head around GN3.